The present invention relates to the field of gas turbine, more specifically it concerns an air-cooled turbine blades.
Air-cooled turbine blades have been disclosed, for example, in U.S. Pat. No. 5,482,435 and U.S. Pat. No. 5,785,496.
Modern gas turbine work at extremely high temperatures. This requires intensive cooling of the turbine blades which are used nowadays in modern gas turbines. In this case, it is usually especially difficult to effectively cool the exposed regions of the blades. One of these region is the shroud band or shroud-band element of the blade. one possibility of cooling the shroud-band element has been described in U.S. Pat. No. 5,785,496. In this publication, it is proposed (see FIGS. 1A and 1B there) to cool the shroud-band element by a number of parallel cooling holes which extend from the (central) moving blade through the shroud-band element to the outer edge of the shroud-band element and open there into the exterior space. In U.S. Pat. No. 5,482,435, only two holes running in opposite directions are provided for the same purpose.
However, these known solutions have disadvantages: the known cooling holes take up comparatively little space inside the shroud-band element. Since a certain minimum thickness of the shroud-band element is required for making the holes in the shroud-band element, and this thickness or an even greater thickness of the shroud-band element is also maintained in the region outside the holes, this results in an unfavorably small ratio of shroud-band volume through which flow occurs to shroud-band volume through which flow does not occur. The result of this is that the cooling of the shroud-band element is not optimal, and that the shroud-band element is comparatively heavy on account of the large proportion of solid material and is thus exposed to high mechanical loads during operation on account of the centrifugal forces.
To solve this problem, it has already been proposed (GB-A-2,290,833) to virtually completely dispense with cooling holes running in the interior of the shroud-band element and to instead cause cooling air to flow like film cooling out of a distribution passage via a number of small openings to the top side of the shroud-band element in order to permit a thinner and lighter shroud-band element. A problem in this case, however, is that the effectiveness of this surface film cooling of the shroud-band element greatly depends on the flow conditions prevailing on the top side of the shroud-band element and can therefore only be optimized with difficulty for the various operating states.
The object of the invention is therefore to provide a turbine blade having an air-cooled shroud-band element, in which turbine blade the abovementioned disadvantages can be avoided in a simple manner and which is characterized by effective cooling of the shroud-band element in particular with a marked reduction in the weight of the shroud-band element.
The essence of the invention is to design the hollow spaces carrying the cooling fluid in the interior of the shroud-band element so as to match the shroud-band element in shape and dimensions in such a way that the volume through which the cooling fluids flows takes up a high proportion of the total volume of the shroud-band element. In this way, the weight of the shroud-band element can be considerably reduced with at the same time very efficient cooling.
A first preferred embodiment of the turbine blade according to the invention is characterized by the fact that the hollow spaces comprise cooling holes, that the cooling holes are of tunnel-shaped design, the thickness of the shroud-band element being reduced outside the cooling holes, and that the cooling holes run from inside to outside essentially parallel to the direction of movement of the blade tip and in each case open upward into the exterior space upstream of the outer margin of the shroud-band element. The tunnel-shaped design of the cooling holes not only reduces the proportion of solid material at the shroud-band element but at the same time stiffens the shroud-band element mechanically. The cooling air discharging at the top can discharge without hindrance even when the shroud-band elements of all the blades of a turbine stage are lined up in sequence and combined to form an annular shroud band.
To this end, recesses are preferably made in the shroud-band element from the top side, and the cooling holes open laterally into the recesses. Furthermore, it is advantageous if a choke point for limiting the cooling-air mass flow is provided in each of the cooling holes, and the choke points are each arranged at the inlet side of the cooling holes. Some of the cooling holes may also be designed as diffusers.
A second preferred embodiment of the invention is characterized in that the hollow spaces are designed as slits which extend over the width of the shroud-band element, in that the slits run from inside to outside essentially parallel to the direction of movement of the blade tip and in each case open upward into the exterior space upstream of the outer margin of the shroud-band element, in that recesses are made in the shroud-band element from the top side, and in that the slits open laterally into the recesses. The wide slits result in good cooling with at the same time a considerable reduction in material. In this case, too, it may be advantageous to provide choke points for limiting the cooling-air mass flow in each of the slits, the choke points each being arranged at the inlet side and/or at the outlet side of the slits.
The cooling is especially effective if, in a preferred development of this embodiment, means of improving the heat transfer between cooling air and shroud-band element are provided in the slits. In particular, the slits may comprise a distributed arrangement of pins as a means of improving the heat transfer, the cooling fluid flowing around these pins in a turbulent manner, and the pins thus further improving the heat transfer between cooling fluid and shroud-band material.
A third preferred embodiment of the turbine blade according to the invention is characterized in that the hollow spaces comprise cooling holes extending in the direction of movement of the blade tip, in that a plurality of transverse holes cross the cooling holes, and in that the transverse holes are blocked off toward the exterior space by closed ends. This configuration of the crossing cooling holes is comparable in geometry to the abovementioned wide slits with distributed pin arrangement. Here, too, with greatly improved heat transfer, the solid material of the shroud-band element is considerably reduced and thus weight is saved. The crossing cooling holes are comparatively easy to make in the shroud-band element with conventional means. Cooling holes which are especially favorable from the cooling point of view can be obtained if the cooling holes and the transverse holes are produced by means of the so-called STEM drilling process.